Microcolumn separation techniques, in particular capillary electrophoresis has become a very interesting separation technique which is used as part of a sensor or a chemical analysis system. One major reason for this is the great efficiency of the method as a separation technique. The sampling methods usually applied in capillary electrophoresis are:
injection of a sample with a syringe, via a septum, in an injection block,
the use of injection valves with/without a sample loop, and
dipping one end of the capillary tube into the sample reservoir, whereby the sample is introduced by gravity flow, by over- or underpressure, or by electroendosmosis and/or electromigration.
While it is mentioned in Journal of Chromatography, 452, (1988) 612-622, that sample valves are the most suitable sampling method for capillary electrophoresis, there also is described a valveless device for the injection of a sample. The described arrangement comprises a cast capillary block which is connected between an electrode compartment and a sampling device. In the electrode compartment electrolyte solutions contact electrodes. The capillary tube contains measuring electrodes which are connected with an evaluation electronics. The sampling device consists of a broadened part of the capillary tube connected with two feeders which extend perpendicular to the capillary tube. The arrangement of the two feeders off-set from each other along the longitudinal extension of the capillary tube is such, that the sampling device has the shape of a capillary double T structure.
The sample is introduced into the sampling device via a syringe. The injection volume is defined geometrically by the distance which the two feeders are spaced apart along the capillary tube. The transport of the electrolyte solution and the sample in the capillary tube is accomplished by electric fields that are applied between the respective electrodes along the capillary tube. An advantage of the double T shape sampling device, as is also obtained with the use of injection valves, is the concentration effect of dilute sample ionic species. However, it is possible that, although no electric field gradient over the feeders exists, sample components from the feeders may diffuse into the capillary tube when the sample has already left the sampling position. The amounts of sample components that uncontrollably enter the capillary tube depend on the diffusion coefficients and the mobilities of the respective sample components. Thus, at the detector there not only arrives a more or less broadened plug of injected sample fluid, depending on the diffusion coefficients and the mobilities of the respective components in the electrolyte and the electric field, but also the electrolyte in front and after or between individual plugs of sample fluid is xe2x80x9cpollutedxe2x80x9d with unpredictable amounts of sample components. These unpredictable amounts of sample components reaching the detector are highly undesirable and result in a high noise of the detected signal, thus reducing the limits of detection considerably.
In Analytical Chemistry, 1992, 64, pages 1926-1932 a capillary electrophoretic device is described in which the sample is injected electrokinetically dipping one end of a capillary into the sample reservoir and applying a voltage across the ends of the capillary. In the electric field the sample is transported electrokinetically and is injected at a T-junction into the channel system of the capillary electrophoretic device. This method, however, leads to a well-known bias of the actual sample composition due to the differences in the electrophoretic mobilities of the sample components. Thus, the sample introduced often does not have the same composition as the original sample. In addition, the volume of the introduced sample is very often unknown such, that internal standards have to be used for quantitative analyses.
It is therefore an object of the present invention to provide a method for controlling sample introduction in microcolumn separation techniques, and more particularly in capillary electrophoresis (CE), and a sampling device which overcomes the aforementioned disadvantages of the prior art The sample volume shall be geometrically defined. The composition of the sample which is injected shall not differ from the original composition of the sample in the reservoir. The uncontrolled introduction of sample fluid into the capillary tube shall be reduced considerably. If the unwanted leakage of sample fluid into the capillary tube cannot be totally avoided, provisions shall be made that at least it only occurs in a predictable and controllable manner.
The method and the sampling device according to the invention shall also allow an easy realization of miniaturized analysis concepts, such as the ones described, for example, in Sensors and Actuators B, 10 (1993) 107-116. There the concept of a multi-manifold flow system integrated on a silicon substrate, with valveless switching of solvent flow between channels and electro-kinetic pumping of an aqueous solvent, is described. A similar concept is described, for example, in Analytical Chemistry , Vol. 64, No. 17, Sep. 1, 1992, 1926-1932. The described miniaturized chemical analysis system on the basis of capillary electrophoresis comprises a complex manifold of capillary channels, which are micromachined in a planar glass substrate. The transport of the solvent and the sample occurs due to electro-kinetic effects (electro-osmosis and/or electrophoresis).
In order to meet all these and still further objects according to the invention a method for controlling sample introduction in microcolumn separation techniques, in especially in capillary electrophoresis (CE) is provided, wherein an electrolyte buffer and a more or less concentrated sample are transported through a system of capillary channels. The sample is injected as a sample plug into a sampling device which comprises at least a channel for the electrolyte buffer and a supply and drain channel for the sample. The channel for the electrolyte buffer and the supply and drain channels for the sample intersect each other. The supply channel and the drain channel for the sample, each discharge into the channel at respective supply and drain ports. The distance between the supply port and the drain port geometrically defines a sample volume. The supply and the drain channels each are inclined to the electrolyte channel. The injection of the sample plug into the electrolyte channel is accomplished electro-kinetically by applying an electric field across the supply and drain channels for a time at least long enough that the sample component having the lowest electrophoretic mobility is contained within the geometrically defined volume. By this measure the composition of the injected sample plug will reflect the actual sample composition.
In a further preferred process step, immediately after the injection of the sample plug, the electrolyte buffer is allowed to advance into the supply channel and into the drain channel at the respective supply and drain ports for a time period, which amounts to at least the migration time of a slowest component within the sample plug from the supply port to the detector. Thus, the sample is pushed back into the respective supply and drain channels and substantially prevented from uncontrollably diffusing into the electrolyte buffer which is transported past the supply and drain ports. In addition the method allows to control the sample composition within the electrolyte buffer.
The sampling device according to the invention comprises an electrolyte channel, and a supply channel and a drain channel for the sample, which discharge into the electrolyte channel at respective supply and drain ports. The ports are arranged with respect to each other such, that a sample volume is geometrically defined. The supply and drain channels each are inclined to the electrolyte channel. Means are provided for electro-kinetically injecting a sample into the sample volume. The resistance to flow of the supply and drain channels with respect to the electrolyte buffer is at least about 5% lower than the respective resistance to flow of the electrolyte channel. Preferred variants of the method according to the invention and preferred embodiments of the sampling device according to the invention are subject of the respective dependent claims.